1. Field of the Invention
The subject invention relates to a heat pump system operable in a heating mode and a cooling mode.
2. Description of the Prior Art
Heat pumps have been utilized to heat and cool structures for many years. The heat pump includes a vapor compression system including a compressor, a condenser, an evaporator, an expansion device, and a refrigerant circulating through the system. In addition to the vapor compression system, the heat pump includes a reversing valve for reversing the flow of the refrigerant within the system. The heat pump removes heat from within the structure when the refrigerant is circulating in a direction, and adds heat to the structure when the circulation of the refrigerant is reversed. The condenser and the evaporator are both heat exchangers, with the refrigerant dissipating heat in the condenser and the refrigerant absorbing heat in the evaporator. The condenser and the evaporator each include at least one air movement device, such as a fan, to increase the airflow over the condenser and the evaporator to increase the operating efficiency of each.
When the heat pump is operating in the cooling mode, the condenser receives the refrigerant from the compressor in a vapor state. As the refrigerant circulates through the condenser, heat stored in the refrigerant is dissipated into the airflow passing across the condenser, thereby cooling the refrigerant. As the refrigerant cools in the condenser, it changes from the vapor state to a liquid state. The refrigerant, in the liquid state, moves from the condenser to the expansion device, where the pressure of the refrigerant is lowered to facilitate evaporation of the liquid refrigerant in the evaporator. The evaporator receives the liquid refrigerant from the expansion device at the lowered pressure. The airflow passes over the evaporator, where the refrigerant absorbs heat from the airflow, thereby evaporating the refrigerant and increasing the temperature of the refrigerant. The heated refrigerant, in the vapor state, circulates into the compressor, where the compressor compresses the vapor, thereby increasing the pressure of the vapor refrigerant to facilitate the phase change from the vapor state to the liquid state in the condenser. Additional heat is added to the refrigerant by the compressor during compression of the refrigerant. Therefore, the condenser must dissipate the heat in the refrigerant absorbed at the evaporator as well as the heat added to the refrigerant by the compressor. Accordingly, the heat exchanger operating as the condenser in the vapor compression system must have a heat transfer capacity higher than that of the heat exchanger operating as the evaporator in the vapor compression system.
When the reversing valve changes the direction of the refrigerant in the system to switch from the cooling mode to the heating mode, the condenser in the cooling mode becomes the evaporator in the heating mode, and the evaporator in the cooling mode becomes the condenser in the heating mode. The vapor compression system operates in the same manner as described above for the cooling mode. Accordingly, the heat transfer capacity of the two heat exchangers (the condenser and the evaporator) is reversed, and the heat exchanger operating as the evaporator in the heating mode may introduce more heat into the vapor compression system than the heat exchanger operating as the condenser is capable of dissipating. This results in an imbalance in the heat transfer rate between the evaporator and the condenser, undermining the system capacity and performance.
U.S. Pat. No. 5,782,101 to Dennis (the '101 patent) discloses a heat pump system operating in the heating mode as described above. The heat pump system further includes a sensor operatively connected to an evaporator fan. The sensor senses the temperature or the pressure of the refrigerant and sends a signal to the evaporator fan. The evaporator fan controls the airflow over the evaporator, thereby controlling the heat transfer rate of the evaporator. Accordingly, when the heat pump is operating in the heating mode and the temperature or the pressure of the refrigerant becomes too high, the sensor signals the fan to slow or disengage to reduce the airflow across the evaporator and limit the heat transferred to the refrigerant at the evaporator. As a result, in order to maintain a required mass flow rate of refrigerant circulating through the vapor compression system, the heat pump of the '101 patent reduces the velocity of the refrigerant circulating through the evaporator. The lower velocity of the refrigerant circulating through the evaporator lowers the efficiency of the heat pump system.